Vector control unit for an induction motor

ABSTRACT

A vector control unit that has an inverter connected with an induction motor which is not only electrically driven but is also being operated in a regenerative condition. The vector control unit comprises a detection circuit for detecting that the induction motor is being operated at a low speed and for detecting that signs of a rotor electrical angular velocity command and a slip angular velocity command differ from each other, and a control circuit for performing control such that the slip angular velocity command is not included in the angular velocity command when the induction motor is being operated at low speed and the signs of the rotor electrical angular velocity command and the slip angular velocity command differ from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vector control unit for an induction motor connected with an inverter, the motor being not only electrically driven but also working in a regenerative condition.

2. Description of the Related Art

A slip frequency control method characterized by both excellent responsiveness and accuracy in control has been known as a variable speed control method for an induction motor. Particularly, a vector control method which can provide responsiveness equivalent to that of a D.C. motor, by controlling a primary current of the induction motor, dividing it into an excitation current and a torque current, and controlling a secondary magnetic flux and the torque current in such a way that their directions are constantly kept in a perpendicular relationship to each other, has been put into force. Recently, a speed sensor has been removed from the method (hereinafter referred to as a PG-less method) and the method has been improved through simplification and improving resistance against adverse environmental conditions.

A PG-less induction motor control unit usually comprises, as shown in FIG. 1, a converter unit which consists of a diode and a capacitor and which converts an electric current from an A.C. source to D.C., a voltage type PWM inverter 1 consisting of an inverter unit for generating A.C. voltage by modulating voltage commands of U-, V-, and W-phase output from a current controller to PWM signals by means of a switching element such as a thyristor or IGBT; electric current detectors 10₁, 10₂, 10₃ for detecting electric currents which flow in U-, V-, and W-phases, respectively, of an induction motor 2; a voltage detector 11 for detecting voltages between two of the U-, V-, and W-phases; a vector control unit 3 for performing vector control; and a command generator 19.

FIG. 2 is a block diagram showing the vector control unit 3 shown in FIG. 1.

The vector control unit 3 is composed of a coefficient meter 4; integrator 5; a function generator 6 which inputs phase θψ* and generates exp (jθψ*), i.e., cos θψ*+jsin θψ*; a two-phase/three-phase converter 7 which converts a vector having components in the direction of a magnetic flux (hereinafter referred to as a "d axis") and in the direction perpendicular thereto (hereinafter referred to as a "q axis") into a vector having components in the directions of the U-, V-, and W-phases which have phase differences of 120° from one another; a vector operation unit 8 for performing the operation of a vector of r=α+jβ which represents a d-axis component α and a q-axis component β, that is, an amplitude |r|=(α² +β²)^(1/2) and a phase tan⁻¹ (β/α); a vector rotator 9 which inputs vector r and exp (jθψ*) and arranges its phase to θψ*+tan⁻¹ (β/α); a magnetic flux operation unit 12 for detecting a magnetic flux and a torque current by using a primary voltage vector v_(l) obtained from a voltage detector 11 and a primary current vector i₁ obtained from the current detectors 10₁, 10₂, 10₃ ; a velocity presuming unit 13 for presuming an electrical angular velocity of a rotor by using a torque current detection value Iτ obtained by the magnetic flux operation unit 12 and a torque current command Iτ*; a magnetic flux command generator 14 which performs field-weakening control based on the magnitude of the electrical angular velocity of the rotor; subtracters 15₁, 15₂ ; a velocity controller 16 for eliminating the error between an angular velocity command ωr* from a command generator 19 and a presumed angular velocity ωr to perform PI control; a magnetic flux controller 17 for eliminating the magnetic flux error Δψ₂ between a magnetic flux command ψ₂ * and a detected magnetic flux ψ₂ to perform PI control; electric current controllers 18₁, 18₂, 18₃ provided for every U-, V-, and W-phase for eliminating the error between a command value and a detected value of a primary current to perform P control; subtracters 20₁ to 20₅ ; and an adder 21. The torque current command Iτ* is obtained by dividing a torque command T* to be acquired as an output of the velocity controller 16 by the magnetic flux command ψ₂ * to be obtained as an output of the magnetic flux command generator 14. An excitation current command Iψ* is obtained as an output of the magnetic flux controller 17.

The operation of the present conventional example will next be described.

When motor constants are expressed by an asymmetrical T type equivalent circuit shown in FIG. 3, the relation between the voltage and the electric current of the induction motor 2 is given by expression (1) in a static coordinate system. ##EQU1## R: resistance of each phase L_(l), M: self-inductance and mutual inductance

l: total leakage inductance (=L_(l) -M)

ω_(r) : angular velocity

p: differential operator

subscript₁,2 : primary and secondary

Further, secondary interleakage magnetic flux ψ₂ and an excitation current i.sub.ψ are represented by expression (2) and (3), respectively.

    ψ.sub.2 =M(i.sub.1 +i.sub.2)                           (2)

    i.sub.ψ =i.sub.1 +i.sub.2                              ( 3)

Expression (1) is developed into expressions (4) and (5) by using expressions (2) and (3).

    v.sub.l =(R.sub.1 +l·p)·i.sub.1 +p·ψ.sub.2( 4)

    O=R.sub.2 ·i.sub.2 +(p-jω.sub.r)·ψ.sub.2( 5)

Next, when a unit magnetic flux vector is represented by Θ.sub.ψ on a magnetic flux rotating coordinate system, a primary current i₁ and a secondary current i₂ are represented by expressions (6) and (7), respectively.

    i.sub.1 =(I.sub.ψ +jIτ)·Θ.sub.ψ ( 6)

    i.sub.2 =-(1/R.sub.2)·{p·ψ.sub.2 +j(ω.sub.ψ -ω.sub.r)·ψ.sub.2 }·Θ.sub.ψ( 7)

where, Θ.sub.ψ =exp(jθ.sub.ψ), θ.sub.ψ : angle of magnetic flux vector, ω.sub.ψ : magnetic flux angular velocity.

In expression (6), the operation of the commands I.sub.ψ +jI.sub.τ and Θ.sub.ψ is performed by the vector operation unit 8 and by the function generator 6, respectively. The vector rotator 9 inputs these two data values, performs the operation of a primary current command i₁ *, and outputs it as a command corresponding to expression (6).

Further, the relation between the torque current command Iτ* and a slip angular velocity command ω_(s) * is represented by the next expression (8), and its operation is executed by the divider 15₂ and the coefficient meter 4.

    ω.sub.s *=R.sub.2 *·Iτ*/ψ.sub.2     ( 8)

An angle command θψ* of a magnetic flux vector is obtained from expression (9) by integrating the sum of a presumed angular velocity ωr output from the velocity presuming unit 13 and the slip angular velocity command ωs* obtained from expression (8) by using integrator 5.

    θψ*=∫ωψ*dt=(ωrωs*)/p  (9)

The primary current command i₁ * output from the vector rotator 9 is converted into each current of U-, V-, and W-phase by the two-phase/three-phase converter 7, and the differences between respective electric currents of the U-, V-, and W-phases and the detected values of phase currents detected by the electric current detectors 10₁, 10₂, 10₃ are inputted into electric current controllers 18₁, 18₂, 18₃, respectively, following which the P-controlled results of the above inputted values are sent out to the voltage type PWM inverter 1 as voltage commands thereto. The voltage type PWM inverter 1 modulates these voltage commands to PWM signals to output to the induction motor 2. In addition, voltages between two of the U-, V-, and W-phase are detected by the voltage detector 11 and inputted into the magnetic flux operation unit 12 together with the detected value of the primary electric current. The magnetic flux operation unit 12 performs the operation of expression (10) for obtaining the magnetic flux vector ψ₂ by compensating a voltage drop due to the primary resistance and leakage inductance from the integral value of an inputted primary voltage v_(l), and performs the operation of expression (11) for obtaining a torque τ. ##EQU2## Im: imaginary part operation mark ψ₂ : conjugate multiple vector of ψ₂

P: number of motor poles

After detecting the amplitude of ψ₂ of expression (10) as the magnetic flux detection value ψ₂, the torque current detection value Iτ is obtained by dividing expression (11) by ψ₂.

In this way, the PG-less vector control is performed. With the above conventional technique, however, there has been a problem that, when a load is applied to the induction motor while it is being operated at a low speed (a regenerative condition), the absolute value of the magnetic flux angular velocity ωψ is reduced, thereby lowering the accuracy of magnetic flux detection by the vector control unit and reducing the operational stability of the vector control unit and also causing the unit to tend to lose synchronism of operation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a vector control unit for an induction motor of the kind described at the beginning in the specification which stabilizes the operation of the induction motor when the motor is in a regenerative condition at a low speed.

According to one aspect of the present invention, there is provided a vector control unit for an induction motor comprising a detection means for detecting that the induction motor is being operated at a low speed and for detecting that the sign of a rotor electrical angular velocity command and the sign of a slip angular velocity command differ from each other, and a control means for performing control such that slip angular velocity command is not included in the magnetic flux angular velocity command when said induction motor is being operated at a low speed and the sign of the rotor electrical angular velocity command and the sign of the slip angular velocity command differ from each other.

According to another aspect of the present invention, there is provided a vector control unit for an induction motor comprising a detection means for detecting that the induction motor is being operated at a low speed and that the sign of the rotor electrical angular velocity command and the sign of the slip angular velocity command differ from each other, and a control means for subtracting the slip angular velocity command from an ordered rotor electrical angular velocity command to output the result of subtraction as a new rotor electrical angular velocity command when said induction motor is being operated at a low speed and the sign of the rotor electrical angular velocity command and the sign of the slip angular velocity command differ from each other.

When the induction motor is operated at a low speed and in a regenerative condition (the sign of the angular velocity command ωr* differs from the sign of the slip angular velocity command ωs*), by not incorporating the slip angular velocity command ωs* in the magnetic flux angular velocity command ωψ*, the magnetic flux angular velocity command ωψ* with a greater absolute value than it originally should have is given to the motor, and by increasing the rotational speed of the induction motor, the absolute value of the magnetic flux angular velocity ωψ is maintained without being lowered, and thus the operational stability of the vector control in the PG-less system is greatly increased. Further, by subtracting the slip angular velocity command ωs* from the angular velocity command ωr* and outputting the result of subtraction as a new electrical angular velocity command ωr*, the angular velocity command ωr* with an absolute value which is larger by the absolute value of the slip angular velocity command ωs* is given to the motor, and accordingly, the rotational speed of the induction motor is increased and the absolute value of the magnetic flux angular velocity ωψ is maintained without being lowered, and thus the operational stability of the vector control in the PG-less system is greatly increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a conventional example of a PG-less system;

FIG. 2 is a block diagram showing a vector control unit 3 of FIG. 1;

FIG. 3 is a view showing an asymmetrical T type equivalent circuit of an induction motor;

FIG. 4 is a block diagram of a vector control unit of a first embodiment of the present invention;

FIG. 5 is a circuit diagram of a detection circuit 40 and switch 30 of FIG. 4; and

FIG. 6 is a block diagram of a vector control unit of a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED PREFERRED EMBODIMENTS OF THE INVENTION

An embodiment of the present invention will next be described with reference to the drawings.

FIG. 4 is a block diagram of a vector control unit for an induction motor according to a first embodiment of the present invention. FIG. 5 is a circuit diagram of switch 30 and a detection circuit 40 shown in FIG. 4. In FIG. 4, the same components as shown in FIG. 2 are given the same reference numbers.

The present embodiment differs from the conventional example (FIG. 2) in that the embodiment includes switch 30 provided between a coefficient meter 4 and an adder 21 for adding a presumed angular velocity ωr to a slip angular velocity command ωs*, and further includes a detection circuit 40 for performing on-off control of switch 30.

As shown in FIG. 5, the detection circuit 40 consists of a multiplier 41, an absolute-value circuit 42, comparators 43₁, 43₂, and an AND circuit 44. The multiplier 41 calculates the product of an angular velocity command ωr* and a torque current command Iτ*. The absolute-value circuit 42 outputs the absolute value of a magnetic flux angular velocity command ωψ*. The comparator 43₁ compares the product of the angular velocity command ωr* and the torque current command Iτ* with 0, and when the product is smaller than 0 (the regenerative condition), the comparator outputs a high-level signal, and when the product is larger than 0 (an electrically driven condition), the comparator outputs a low-level signal. Comparator 43₂ compares |ωψ*| with a reference value, and when |ωψ*| is smaller than the reference value, the comparator outputs a high-level signal, and when |ωψ*| is larger than the reference value, the comparator outputs a low-level signal. The AND circuit 44 inputs the outputs of both comparators 43₁, 43₂, and outputs the high-level signal when the induction motor 2 is in the regenerative condition and the magnetic flux angular velocity command ωψ* is below the reference value in order to connect terminal 30₃ of the switch 30 to terminal 30₂ (ground voltage). Otherwise, terminal 30₃ is connected to terminal 30₁, resulting in the same state as established with the conventional example.

Therefore, when the motor is operated in the regenerative condition at a low speed, the slip angular velocity command ωs* is not incorporated in the magnetic flux angular velocity command ωψ*, with the result that the magnetic flux angular velocity command ωψ* made to have an absolute value larger than its original value is given to increase the rotational speed of the induction motor 2, and hence the absolute value of the magnetic flux angular velocity ωψ is maintained without being lowered, thereby increasing of the stability of the PG-less system.

FIG. 6 is a block diagram of a vector control unit for an induction motor with reference to a second embodiment of the present invention.

The present embodiment comprises a switch 30 provided between the output side of a coefficient meter 4 and the input side of a velocity controller 16, and when the induction motor 2 is operated in the regenerative condition and the magnetic flux angular velocity command ωψ* is below the reference value, the switch 30 is operated by the detection circuit 40 to connect terminal 30₃ to terminal 30₁, and a new velocity command ωr* is obtained by subtracting the slip angular velocity command ωs* from the velocity command ωr* by using a subtracter 20₆. In other words, by increasing the absolute value of the velocity command ωr* by the absolute value of the slip angular velocity command ωs* to increase the rotational speed of the induction motor 2, the absolute value of the magnetic flux angular velocity ωψ is maintained without being lowered, thereby increasing stability of the vector control of the PG-less system. In the present embodiment, switch 30 and subtracter 20₆ constitute the control means.

In both the first and second embodiments, the torque current command Iτ* is used as one of means for judging the regenerative condition; however, since the values of the magnetic flux command ψ₂ * and the coefficient meter 4 are always positive, quite the same result is obtained compared with the result which can be obtained by using the slip angular velocity command ωs*.

Further, as an input signal to the detection circuit 40, the presumed angular velocity ωr can be used instead of the angular velocity command ωr*, and a torque current detection value Iτ can be used instead of the torque current command Iτ*.

Furthermore, although examples of the conventional type and the embodiments of the invention are described with reference to the cases in which they are applied to the slip frequency type PWM inverter control unit, it is also possible to apply the present invention to a PWM inverter control unit of a magnetic flux phase standard type.

While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims. 

What is claimed is:
 1. A vector control unit for an induction motor connected with an inverter, the motor being not only electrically driven but also working in a regenerative condition, comprising:detection means for detecting that said induction motor is being operated at a low speed and for detecting that the sign of a velocity command and the sign of a slip angular velocity command differ from each other; control means for performing control such that the slip angular velocity is not included in the magnetic flux angular velocity command when said induction motor is being operated at a low speed and the sign of the velocity command and the sign of the slip angular velocity command differ from each other.
 2. The vector control unit according to claim 1, wherein,said detection means comprises a multiplier for calculating the product of a velocity command ωr* and a torque current command Iτ*, an absolute value circuit for outputting the absolute value of a magnetic flux angular velocity command ωψ*, a first comparator for comparing the product of the velocity command ωr* and the torque current command Iτ* with zero, a second comparator for comparing the absolute value of the magnetic flux angular velocity command ωψ* with a reference value, a gate circuit which inputs outputs of the first and second comparators and outputs a signal of a predetermined logical level when the product of the velocity command ωr* and the torque current command Iτ* is less than zero and the absolute value of the magnetic flux angular velocity command ωψ* is smaller than the reference value; said control means is a switch provided in the former stage of an adder which adds the slip angular velocity command ωs* to a presumed velocity ωr to output the magnetic flux angular velocity command ωψ*, and said switch outputs said slip angular velocity command ωs* to said adder when said predetermined logic level signal is not outputted from said gate circuit and outputs zero to said adder instead of said slip angular velocity command ωs* when said predetermined logic level signal is outputted.
 3. The vector control unit for an induction motor connected with an inverter, the motor being not only electrically driven but also working in a regenerative operation, comprising:detection means for detecting that said induction motor is being operated at a low speed and for detecting that the sign of a velocity command and the sign of a slip angular velocity command differ from each other; control means for subtracting the slip angular velocity command from an ordered velocity command to output the result of subtraction as a new velocity command when said induction motor is being operated at a low speed and the sign of the velocity command and the sign of the slip angular velocity command differ from each other.
 4. The vector control unit according to claim 3, wherein,said detection means comprises a multiplier for obtaining the product of a velocity command ωr* and a torque current command Iτ*, an absolute value circuit for outputting the absolute value of a magnetic flux angular velocity command ωψ*, a first comparator for comparing the product of the velocity command ωr* and the torque current command Iτ* with zero, a second comparator for comparing the absolute value of the magnetic flux angular velocity command ωψ* with a reference value, a gate circuit which inputs outputs of the first and second comparators and outputs a signal of a predetermined logical level when the product of the angular command ωr* and the torque current command Iτ* is less than zero and the absolute value of the magnetic flux angular velocity command ωψ* is smaller than the reference value; said control means is a switch provided in the former stage of a subtracter which subtracts a slip angular velocity command ωs* from a presumed velocity command ωr and outputs zero to said subtracter instead of said slip angular velocity command ωs* when said predetermined logic level signal is not outputted from said gate circuit and inputs said slip angular velocity command ωs* to said subtracter when said predetermined logic level signal is outputted from said gate circuit. 